The invention relates to methods and apparatus for preventing exhausted reactant gas from mixing with ambient air of a semiconductor wafer fabrication facility and thereby reducing corrosion of "scavenger boxes" in the semiconductor wafer fabrication facility, improving the safety for employees, and improving the uniformity and/or repeatability of semiconductor wafer processing reactions within reaction chambers or furnaces.
Most furnace processing operations in the semiconductor industry are performed at or near ambient atmospheric pressure and are affected by changes in exhaust pressure. However, exhaust pressure varies considerably due to a wide range of factors, including demands made on the exhaust system by other pieces of equipment coupled into the exhaust network, changes in processes from tube to tube within the furnace, and changes in the process occurring within the tube. In some cases, the amount of scavenger exhaust draw required in order to prevent toxic or corrosive gases from escaping the scavenger is so high that the process is adversely affected, even if the exhaust pressure is unchanging. These changes in exhaust pressure result in variations in the gas flow pattern of process gas flowing through wafers such as 20 in FIG. 1. These gas flow variations result in non-uniformities and, more importantly, in non-repeatability in the process reaction results on the wafers. This in turn results in reduction in wafer yield, which can be very expensive.
The above mentioned "scavenging systems" in the semiconductor industry usually include rectangular metal inlets, roughly 12 inches square, into which spent reaction gasses are drawn and routed to the exhaust "scrubber" system of the wafer fabrication facility. The toxic gasses usually cause corrosion of the scavenger boxes and nearby electrical and mechanical components. The corrosion generates airborne particulate which are very undesirable in a semiconductor processing facility in which the air should be ultraclean.
The health of operators and employees also maybe affected by any spent reactant gases which escape the scavenger and flow into the air of wafer fabrication facility (often referred to simply as the "fab").
Low pressure chemical vapor deposition (LPCVD) systems are known in which gas pressures in wafer processing chambers are nearer to a perfect vacuum; the pressures in LPCVD systems often are in the range from 0.1 to 100 torr. Commercially available absolute pressure sensors having a full scale reading of 0-1, 0-10, or 0-100 torr are used to measure pressure in an LPCVD system. The low pressure is measured and precisely regulated to within approximately 0.5 percent of the full scale value by comparing the measured pressure with a preset device pressure value and controlling the operation of a vacuum pump connected to an outlet port similar to 17 in FIG. 1. Uniformity of processing results produced on the wafers in LPCVD systems, and the repeatability of such results is acceptable, but deposited films such as oxide films vary from system to system, and are subject to pinholes and voids within the deposited film. Conventional LPCVD systems have various disadvantages, including having very slow wafer throughput, requiring very expensive vacuum hardware which is difficult and expensive to maintain, tending to generate excessive particulate, and requiring relatively large spacing between wafers.
Other prior art systems, referred to as high pressure systems, are known in which the semiconductor processing steps, such as oxide growth, are performed at very high pressures, for example 10-25 atmospheres, in order to greatly increase deposition rates or to lower the reaction temperatures. Such high pressure reaction systems usually are used to grow thick "field" oxides. Ordinarily, the thickness of a field oxide is easy to make uniform, so the pressure control systems utilized to maintain the high pressure at a particular value (for example 10 atmospheres) inside the reaction chamber do not provide high precision pressure regulation.
For approximately about the last 30 years, the vast majority of all semiconductor wafer processing operations in closed reaction chambers have been performed at or very close to ambient atmospheric pressure. No one has attempted to precisely control pressure in such atmospheric pressure reaction chambers, despite the non-repeatability and non-uniformity caused by the above mentioned well known variations in exhaust conditions. Perhaps an important reason that no furnace manufacturers have provided a solution to these problems is the difficulty in obtaining equipment capable of accurately measuring absolute pressure to the required accuracy, as is done in the above mentioned LPCVD systems. Furthermore, demand for higher uniformity and more dense circuitry means tighter control of process parameters is required.
Ways of preventing corrosion of the above mentioned scavenger boxes have included running an exhaust tube directly from the reaction gas outlet of the reactor tube directly through the scavenger box into the scrubber inlet, diluting gases in the scavenger, and providing various mechanical barriers. Use of exhaust gas tubes has been found to cause severe problems with wafer processing uniformity and/or repeatability when cantilever tubes of the type described in the assignee's ATMOSCAN system (described in U.S. Pat. No. 4,526,534 "cantilever diffusion tube apparatus and method", issued Jul. 2, 1985 to Wollmann, and U.S. Pat. No. 4,543,059 "slotted cantilever diffusion tube system and method and apparatus for loading", issued on Sept. 24, 1985 to Whang et al, both incorporated herein by reference) are utilized. Dilution gases and barrier methods do not adequately protect the process from variations in exhaust draw.
Toxic exhausted reactant gases, even at low concentrations, are a health hazard and cause discomfort to furnace operators. Therefore, in present wafer fabrication facilities, exhaust flow rates are set at high levels to insure that gases in the scavenger area are drawn into the exhaust system. Such high exhaust gas flow rates or suction levels sometimes deleteriously affect wafer processing results.
Until now there has been an unmet need for providing an apparatus and technique for separating exhaust effluent without subjecting the process tube to effects of exhaust pressure variation, and improving the uniformity and/or repeatability of semiconductor wafer processing reactions performed at approximately atmospheric pressure. There also has been an unmet need for an apparatus and technique for preventing corrosion of scavenger boxes and releasing toxic gases in semiconductor wafer fabrication facilities without causing problems in the uniformity of semiconductor wafer reaction results and/or the repeatability thereof.